.\" Copyright (c) 2008      Sun Microsystems, Inc.  All rights reserved.
.\" Copyright (c) 2011-2012 Cisco Systems, Inc.  All rights reserved.
.TH mpicxx 1 "Jun 10, 2020" "4.0.4" "Open MPI"
.
.SH NAME
mpicxx -- Open MPI C++ wrapper compiler
.
.SH SYNTAX
mpicxx [-showme|-showme:compile|-showme:link] ...
.
.SH OPTIONS
.TP
--showme
This option comes in several different variants (see below).  None of
the variants invokes the underlying compiler; they all provide
information on how the underlying compiler would have been invoked had
.I --showme
not been used.
The basic
.I --showme
option outputs the command line that would be executed to compile the
program.  \fBNOTE:\fR If a non-filename argument is passed on the
command line, the \fI-showme\fR option will \fInot\fR display any
additional flags.  For example, both "mpicxx --showme" and
"mpicxx --showme my_source.c" will show all the wrapper-supplied
flags.  But "mpicxx --showme -v" will only show the underlying
compiler name and "-v".
.TP
--showme:compile
Output the compiler flags that would have been supplied to the
C++ compiler.
.TP
--showme:link
Output the linker flags that would have been supplied to the
C++ compiler.
.TP
--showme:command
Outputs the underlying C++ compiler command (which may be one
or more tokens).
.TP
--showme:incdirs
Outputs a space-delimited (but otherwise undecorated) list of
directories that the wrapper compiler would have provided to the
underlying C++ compiler to indicate where relevant header files
are located.
.TP
--showme:libdirs
Outputs a space-delimited (but otherwise undecorated) list of
directories that the wrapper compiler would have provided to the
underlying linker to indicate where relevant libraries are located.
.TP
--showme:libs
Outputs a space-delimited (but otherwise undecorated) list of library
names that the wrapper compiler would have used to link an
application.  For example: "mpi open-rte open-pal util".
.TP
--showme:version
Outputs the version number of Open MPI.
.TP
--showme:help
Output a brief usage help message.
.PP
See the man page for your underlying C++ compiler for other
options that can be passed through mpicxx.
.
.
.SH DESCRIPTION
.PP
Conceptually, the role of these commands is quite simple:
transparently add relevant compiler and linker flags to the user's
command line that are necessary to compile / link Open MPI
programs, and then invoke the underlying compiler to actually perform
the command.
.
.PP
As such, these commands are frequently referred to as "wrapper"
compilers because they do not actually compile or link applications
themselves; they only add in command line flags and invoke the
back-end compiler.
.
.
.SS Background
Open MPI is comprised of three software layers: OPAL (Open Portable
Access Layer), ORTE (Open Run-Time Environment), and OMPI (Open MPI).
There are wrapper compilers for each layer; each layer's wrapper only
links in the libraries relevant for that layer.  Specifically, each
layer provides the following wrapper compilers:
.
.TP 4
OPAL
\fIopalcc\fR and \fIopalc++\fR
.
.TP
ORTE
\fIortecc\fR and \fIortec++\fR
.
.TP
OMPI
\fImpicc\fR, \fImpic++\fR, \fImpicxx\fR, \fImpiCC\fR (only on systems with
case-senstive file systems), and \fImpifort\fR (and its legacy/deprecated
names \fImpif77\fR and \fImpif90\fR).  Note
that \fImpic++\fR, \fImpicxx\fR, and \fImpiCC\fR all invoke the same
underlying C++ compiler with the same options.  All are provided as
compatibility with other MPI implementations.
.
.
.SS Fortran Notes
.PP
The Fortran wrapper compiler for MPI (\fImpifort\fR, and its
legacy/deprecated names \fImpif77\fR and \fImpif90\fR) can compile and
link MPI applications that use any/all of the MPI Fortran bindings:
.IR mpif.h ,
the
.I mpi
module, and the
.I mpi_f08
module (assuming Open MPI was installed with support for each of these
Fortran bindings).  Specifically: it is no longer necessary to use
different wrapper compilers for applications that use
.I mpif.h
vs. applications that use the
.I mpi
module -- just use
.I mpifort
for all Fortran MPI applications.
.
.PP
Note, however, that the Fortran compiler may require additional
command-line options to enforce a specific Fortran dialect.  For
example, in some versions of the IBM XLF compiler, if xlf90 is the
underlying Fortran compiler,
.IR -qfixed
may be necessary to compile fixed-format Fortran source files.
.
.PP
Finally, note that
.I mpifort
will be inoperative and will return an error on use if Fortran support
was not built into the MP Ilayer.
.
.
.SS Overview
\fImpicxx\fR is a convenience wrappers for the underlying
C++ compiler.  Translation of an Open MPI program requires the
linkage of the Open MPI-specific libraries which may not reside in
one of the standard search directories of ld(1).  It also often
requires the inclusion of header files what may also not be found in a
standard location.
.
.PP
\fImpicxx\fR passes its arguments to the underlying C++
compiler along with the -I, -L and -l options required by Open MPI
programs.
.
.PP
The Open MPI Team \fIstrongly\fR encourages using the wrapper
compilers instead of attempting to link to the Open MPI libraries
manually.  This allows the specific implementation of Open MPI to
change without forcing changes to linker directives in users'
Makefiles.  Indeed, the specific set of flags and libraries used by
the wrapper compilers depends on how Open MPI was configured and
built; the values can change between different installations of the
same version of Open MPI.
.
.PP
Indeed, since the wrappers are simply thin shells on top of an
underlying compiler, there are very, very few compelling reasons
\fInot\fR to use \fImpicxx\fR.  When it is not possible to use the
wrappers directly, the \fI-showme:compile\fR and \fI-showme:link\fR
options should be used to determine what flags the wrappers would have
used.  For example:
.
.PP
shell$ cc -c file1.c `mpicc -showme:compile`
.
.PP
shell$ cc -c file2.c `mpicc -showme:compile`
.
.PP
shell$ cc file1.o file2.o `mpicc -showme:link` -o my_mpi_program
.
.
.SH NOTES
.PP
It is possible to make the wrapper compilers multi-lib aware.  That
is, the libraries and includes specified may differ based on the
compiler flags specified (for example, with the GNU compilers on
Linux, a different library path may be used if -m32 is seen versus
-m64 being seen).  This is not the default behavior in a standard
build, but can be activated (for example, in a binary package
providing both 32 and 64 bit support).  More information can be found
at:
.PP
  https://github.com/open-mpi/ompi/wiki/compilerwrapper3264
.
.
.SH FILES
.PP
The string that the wrapper compilers insert into the command line
before invoking the underlying compiler are stored in a text file
created by Open MPI and installed to
\fI$pkgdata/mpicxx-wrapper-data.txt\fR, where \fI$pkgdata\fR
is typically \fI$prefix/share/openmpi\fR, and \fI$prefix\fR is the top
installation directory of Open MPI.
.
.PP
It is rarely necessary to edit this file, but it can be examined to
gain insight into what flags the wrappers are placing on the command
line.
.
.
.SH ENVIRONMENT VARIABLES
.PP
By default, the wrappers use the compilers that were selected when
Open MPI was configured.  These compilers were either found
automatically by Open MPI's "configure" script, or were selected by
the user in the CC, CXX, F77, and/or FC environment variables
before "configure" was invoked.  Additionally, other arguments
specific to the compiler may have been selected by configure.
.
.PP
These values can be selectively overridden by either editing the text
files containing this configuration information (see the \fBFILES\fR
section), or by setting selected environment variables of the
form "OMPI_value".
.
.PP
Valid value names are:
.
.TP
CPPFLAGS
Flags added when invoking the preprocessor (C or C++)
.
.TP
LDFLAGS
Flags added when invoking the linker (C, C++, or Fortran)
.
.TP
LIBS
Libraries added when invoking the linker (C, C++, or Fortran)
.
.TP
CC
C compiler
.
.TP
CFLAGS
C compiler flags
.
.TP
CXX
C++ compiler
.
.TP
CXXFLAGS
C++ compiler flags
.
.
.TP
FC
Fortran compiler
.
.TP
FCFLAGS
Fortran compiler flags
